The B-2000 was Browning's first gas operated automatic shotgun. They had a bad reputation for reliability but most of the ones that I saw (worked in a Sporting Goods store back then) were either filthy or assembled incorrectly. I bought one that "didn't work" for a song and after a good cleaning it was flawless. The side loading port made it an excellent sporting clays gun. The real purpose of the side port was, when the gun was empty and the bolt locked back, you fed a shell in thru the side port and it automatically chambered, ready to fire.
The B-2000 fit me well, and I shot it well, I wish I still had it.